The present invention relates to a light pick-up device and a light source unit incorporated in the same. In the light pick-up device, laser light from a laser light source is converged onto an optical recording medium through an objective lens, and recording information of the optical recording medium is reproduced by receiving the returning light beam from the optical recording medium by a photo detector.
The light pick-up device to reproduce an optical disk such as a CD or a DVD, is provided with an objective lens driver to converge the laser light for reproducing onto a track position on the target optical disk. The objective lens driver includes a mechanism for movably supporting the objective lens in the optical axis direction of the laser light and in the radius direction of the optical disk, and a mechanism for moving the objective lens supported by the supporting mechanism in the optical axis direction of the laser light and in the radius direction of the optical disk. Generally, the supporting mechanism uses the force of a spring such as a mechanical spring or a magnetic spring, to hold the objective lens at a predetermined neutral position.
For example, there has been proposed an axial sliding type objective lens driver wherein the objective lens is finely moved in the radius direction of the disk by rotating a lens holder holding the objective lens around a rotation axis of the holder.
Being not limited to the above type objective lens driver, generally when the lens holder is moved, the larger inertia force acts on the objective lens when the lens holder is moved in the radius of the disk (at the time of the tracking correction), as compared to when the lens holder is moved in the optical axis direction (at the time of focusing correction). Accordingly, even though the objective lens is tried to stop at a desired position, the lens holder holding the objective lens receives the force oscillating in the radius of the disk, and it is necessary to wait the attenuation of the oscillation of the objective lens until the objective lens becomes stationary at the desired position. Accordingly, there is a problem that the reproduction of the recording information takes a long time period.
In view of the foregoing, the object of the present invention is to provide a light pick-up device a light source unit incorporated in the same by which, even when the objective lens driver is moved in the radius direction of the disk, the objective lens can become stationary at the desired position in a short time period.
Further, in such the light pick-up device, the relative position of the semiconductor laser and the optical element, or the relative position of the reflection mirror and the light detector largely affects the characteristic of the device. Accordingly, the second object is to provide a light pick-up device by which the adjustment of the relative position of each optical element constituting the optical system of the device can be easily carried out.
In order to achieve the above objects, there is provided a light pick-up device comprising:
a laser light source for emitting a laser light beam;
an objective lens for converging the laser light beam on an optical recording medium;
a photo detector for detecting a light beam reflected by the optical recording medium;
a first diffractor for separating the light beam emitted from the laser light source and the light beam reflected by the optical recording medium in it""s different optical paths the first diffractor having a diffraction grating pattern for dividing the reflected light beam in a direction perpendicular to a track direction of the optical recording medium such that an optical axis of the reflected light beam becomes the center of division in order to generate a first divided light beam and a second divided light beam; and
a signal processor for generating a reproduction signal in accordance with the detection of the photo detector, the signal processor including a lens deviation detector for detecting a deviation amount of the objective lens from the optical axis, in accordance with respective illuminances of the first and second divided light beams.
In the device, the deviation amount of the objective lens from the optical axis is detected in the lens deviation detector according to the illuminance of the first and the second divided light beams. Accordingly, when the lens drive mechanism is feedback-controlled according to the detection result of the deviation detector, the objective lens can become stationary at the desired position in a short time. Accordingly, the time period for the oscillation attenuation of the objective lens can be shortened, thereby, a high speed operation of the reproduction of the recording information can be attained.
Herein, the illuminance of the reflected light beam from the optical recording medium fluctuates according to the depth of a plurality of pits forming the track or the shape of edges of the pit. Therefore, preferably, the signal processor includes a peak holder to hold respective peak values of the received illuminance in the first and the second divided light beams. Accordingly, the deviation amount of the objective lens can be accurately detected.
Preferably, the light pick-up device further includes a second diffractor for dividing the light beam emitted from the laser light source into a main beam and two subsidiary beams, in order to detect a tracking error of the objective lens by the 3-beam method. In the device, the reflected light beam of the main beam may be divided by the first diffractor in order to conduct the lens deviation detection.
When the reflected light beam of the main beam is subjected to the division of the first diffractor, the respective illuminances of the divided light beams (the first and second divided light beams) are compared to each other, and by feedback-controlling the position of the objective lens so that these amounts become the same, the objective lens can be made to become stationary in a short time at a position which coincides with the optical axis from the light source.
Preferably, the second diffractor has a diffraction grating pattern which is periodically arranged in a direction slanting a predetermined angle with respect to the track direction. Accordingly, by only attaching the second diffractor to a frame body or a light source unit, the light spot of each beam can be formed at appropriate positions on the track so that the tracking error signal is obtained from two subsidiary beams. Namely, in order to adjust the forming position of the light spot, it is not necessary to adjust the angle of the second diffractor with respect to the optical axis.
The light pick-up device of the present invention can be applied not only to the light pick-up device in which the laser light source or the first diffractor are separately assembled to the frame, but also to the light pick-up device in which these are assembled into a common package body to constitute single light source unit. Preferably, the photo detector and the signal processor are provided on a common semiconductor substrate.
In such the device, since the relative position of the semiconductor laser and the optical elements, or the relative position of the reflection mirror and the photo detector largely affects the device performance. Preferably, there is provided a positioning mark on the semiconductor substrate for specifying a mounting position of the laser light source. Utilizing the mark as a sign, the laser light source can be accurately positioned with respect to the photo detector.
In order to easily regulate positions of the front, rear, left and the right of the laser light source on the surface of the substrate, preferably, the positioning mark includes a first pair of marks arranged in a first direction parallel to the optical axis of the laser beam and a second pair of marks arranged in a second direction perpendicular to the first direction.
In order to accurately form the positioning mark on the surface of the substrate by a simple operation, the positioning mark may be simultaneously formed at the time of exposure by which the photo detector is formed on the surface of the semiconductor substrate.
In the device, when the respective illuminances of the first and second divided light beams are designed to be the same under the condition that the first diffractor is correctly positioned, the off-set amount of the first diffractor can be detected from the difference between the illuminances of the two divided light beams. Accordingly, the position of the first diffractor may be adjusted to a position at which the off-set amount becomes zero. As described above, according to the present invention, the position deviation of the first diffractor can be clearly found, thereby, the adjustment of its position can be accurately conducted.